In traditional artistic endeavors, an artist creates attractive visual effects by using a paintbrush to create lines, strokes or paths that have different widths along its length. The artist would traditionally accomplish this by varying the pressure, or the angle, of the moving brush. In computer graphics, variable line widths are not easy to achieve and when achieved are not easily changed.
For the most part existing graphics systems allow users to construct constant width paths by moving a pen (or mouse) over a pad. Such existing graphics software systems generally approximate varying width lines by forcing the user to create a path which describes the width boundaries of the desired stroke. The system then fills in between the boundaries to create the image of the variable width line.
Currently there are two major divisions between kinds of graphic software systems, one of which is generally referred to as raster or paint systems, and the other major division being vector based or outline based graphics software systems.
In the raster based systems, images are created by composing thousands of tiny dots. The advantage of this is that it allows for very free expression but it is very hard to edit discrete shapes once the initial line is drawn.
In raster systems, it is possible to approximate a traditional paintbrush effect by using a pressure sensitive pen which creates a range of dots across the screen. The problem is that when the user is finished drawing the line, it is very hard to go back and fine tune the shape because the image consists of a plurality of disconnected dots. Essentially, each created dot must be individually modified to create a different shape.
A vector based system describes shapes by a series of mathematical outlines. For example, a circle would be described as the mathematical outline of a circle along with an instruction that it should be filled. Using traditional vector graphics programs, a user would describe a variable width line but would have to think about what the boundary, i.e., the outline, of that final shape would be. These systems run counter to traditional artistic techniques and do not follow the normal mind flow of an artist.
Accordingly, one problem with the prior art graphics systems is that they are very difficult to use to obtain a traditional artist effect.
A further problem is that existing systems do not allow artists to create images in a manner which parallels their traditional training, i.e., by moving a paintbrush (stylus) over a pad.
Another problem with the prior art is that once a line has been created, it is very difficult to change that line and to create different graphic images of the width characteristics of the line.
A further problem exists when variable width lines are being created by a pressure stylus since at the end of the line the user would normally lift up on the stylus, thereby reducing the pressure. In a logical format then the line at the end would always become very thin because thin lines are a result of low pressure on the stylus. Thus, there is a need for a system which creates wide lines by heavy pressure on a stylus (pen) which allows a user to pick up on the pen at the end of the line and not create a very thin line.